1. Technical Field
The technology disclosed herein relates to an imaging device having a focal point detecting function.
2. Background Information
Digital cameras that make use of a CCD (charge coupled device) image sensor, a CMOS (complementary metal oxide semiconductor) image sensor, or another such imaging element to convert an optical image into an electrical signal and digitize the electrical signal have become very popular in recent years.
Cameras that employ a phase difference detection type of autofocusing function (AF function) are a known type of digital single-lens reflex camera. With a phase difference detection type of autofocusing function, the defocus direction and the defocus amount can be detected without moving the focus lens, so the focus lens can be moved to the focal position and the time it takes for autofocusing can be reduced (see Japanese Laid-Open Patent Application 2007-163545, for example). With a conventional digital single-lens reflex camera, a movable mirror is provided in order to guide an optical image of the subject to the phase difference detection unit. This movable minor is provided so that it can be inserted into and retracted out of the optical path from the lens barrel to the imaging element.
Meanwhile, a contrast detection type of autofocusing function (also called contrast AF), featuring an imaging element and a viewfinder function constituted by an EVF (electronic viewfinder) or an LCD (liquid crystal display) in what is known as a compact digital camera, has been employed to reduce the size of a digital camera (see Japanese Laid-Open Patent Application 2007-135140, for example). This contrast AF directly detects the focus at the imaging plane, so an advantage is that it is generally more accurate than phase difference detection autofocusing.
With the contrast AF discussed in Japanese Laid-Open Patent Application 2007-135140, an evaluation value is calculated from image data produced by an imaging element, and the position of the focus lens at which this evaluation value is greatest is considered to be the focal position.
However, since only the evaluation value obtained from a single imaging element is used as a reference, the focus lens must be driven for a relatively long time to determine an increase or decrease in the evaluation value. Accordingly, contrast AF takes a relatively long time, which means that focal point detection takes longer.